This invention arose initially from the need to be able to produce soft tissue products from western softwoods. Western softwoods produce a rather harsh product. Different pulp species are frequently blended with softwood pulps to improve softness. However for certain papermills, a wide variety of species for blending is not readily available. Purchased pulp can be more costly than pulp manufactured from the more readily available softwoods for such paper mills. Thus, economics dictate that tissue be made mostly from pulp produced on site.
Small improvements can be made to tissue softness by methods such as chemical additions, optimizing creping and other papermaking operations, and sheet post treatments such as embossing. Such methods may not always produce the required softness.
The prior art has recognized that specific mechanical treatments of certain pulps prior to its formation into a sheet can enhance softness. For example, U.S. Pat. No. 4,036,679 to Back et al. discloses a disc-refiner treatment method for treating pulp to improve various properties, including softness. The process employs the feeding of dried pulp of a consistency of approximately 70% to 90% O.D. (oven dried) by weight through a disc refiner. The pulp exiting the refiner is fluffed and fiberized, has increased bulk, decreased tensile strength, increased absorbency, increased freeness, and improved softness. Manipulation of such dried and fluffed pulp is not without drawbacks. Also, there are added costs associated with drying or removing water from pulp to achieve a 70% to 90% consistency.
U.S. Pat. No. 2,516,384 to Hill et al. and an article authored by Hill and others (H. S. Hill, J. Edwards, and L. R. Beath, "Curlated Pulp--A New Approach to Pulp Processing", Paper Trade Journal, pp. 19-27, Mar. 17, 1949) discloses a mechanical pulp treatment process to impart curl and incidently softness using lower consistency pulp than that taught by the Back et al. patent. In the Hill process, pulp at a consistency between 2% and 60% is confined under mechanical pressure between two elements which are in relative gyratory or reciprocal motion. This creates nodules or balls of pulp between the opposed working elements. Although Hill et al. assert that the curl imparted to their fibers was permanent, the effect was determined to be temporary. For example, the Back et al. '679 patent indicates that the fiber modification of Hill et al. was not lasting in nature since a large amount of the twists, kinks, and bends dissipated upon standing over a 24 to 48 hour time period. It was theorized that this was due to the substantial amount of water that surrounds and is contained within the fibers which tends to reduce the amount of lasting structural distortion which might otherwise result. (U.S. Pat. No. 4,036,679, column 1, line 62 through column 2, line 6.) Further, the Hill et al. article indicates that freeness of its product under the best conditions is increased only slightly, and typically decreased after heavy working.
Curlation, kinking and twisting of fibers might also generate improvements in the papermaking process for the finished product apart from increased softness. For example, Hill et al. in their article recognize that curlation enhances dewatering of wet pulp in the wet press or couch section of the papermachine where pressure is applied to squeeze water from the sheet. They also recognize that curlation enhances loss of water vapor upon drying, although none of these effects was quantified. Other improvements might also be realized.